1. Field of the Invention
The present invention relates to a therapy system using a radiation and a method of positioning a patient couch. More particularly, the present invention relates a therapy system using an ion beam, such as a proton or a carbon ion beam, to a tumor (diseased tissue) for treatment, and a couch positioning method.
2. Description of the Related Art
There is known a therapy method of setting an irradiation target center at a tumor, e.g., a cancer, in a patient's body and delivering an ion beam, such as a proton or carbon ion beam, to the tumor. An ion beam therapy system with such a therapy method comprises a beam generator consisting of an accelerator and a beam transport system, and a beam delivery unit mounted to a rotating gantry. Ion beam accelerated by the accelerator reaches the beam delivery unit through the beam transport system, and is monitored and shaped to conform to the shape of the tumor in the patient's body by the beam delivery unit. Ion beam, such as a proton or carbon ion beam, has a characteristic that most of beam energy is released just before the particles of the beam come to rest (The resulting shape of the radiation dose distribution is called a “Bragg peak”). In the particle beam therapy system, kinetic energy of the ion beam is selected so that the ion beam may stop in the irradiation target and deposit most of the its energy to the tumor based on the above-mentioned characteristic.
Prior to irradiation with an ion beam, the tumor must be properly positioned with respect to the beam delivery unit so that the tumor irradiation dose is maximized at the tumor and the damage to normal tissues is minimized.
For that reason, a patient couch is positioned by a patient couch positioning apparatus (see, e.g., U.S. Pat. No. 5,825,845 (JP, A 2000-510023)). In order to accurately position the patient relative to the beam delivery unit by use of the patient couch positioning apparatus, the position of the irradiation target is decided beforehand relative to particular localization points in the patient's body (e.g. bony landmarks) by using two orthogonal display images.
Prior to the irradiation with an ion beam, an X-ray source is inserted into the location of the ion beam path within the beam delivery unit, and an X-ray image receptor (a plurality of X-ray detectors) is inserted on an extension of the beam path on the opposite side of the patient lying on the couch. The X-ray image receptor produces a radiographic transmission image of the patient's body. To align the tumor with the isocenter located on the extension of the beam path, the direction in and the distance by which the patient couch is to be moved relative to the beam delivery unit are determined by using the offset distance on the X-ray image from each of the particular localization points to the center of the X-ray beam and the offset distance on the DRR from the same particular monument to the isocenter. Then, the patient is repositioned using positioning control of the patient couch, based on the thus-determined direction and distance of movement of the patient couch.
A process of computing the direction and the amount of movement of a couch to position the tumor relative to a beam delivery unit for an ion beam by using a control image (reference image) formed from a tomographic image obtained by X-ray CT (computerized tomography) beforehand (before the positioning procedure) and current orthogonal X-ray images obtained by an X-ray source and an X-ray image receptor is described in from line 37, column 8 to line 66, column 9 (particularly (1a)-(1h)) and FIGS. 3, 4 and 5 of U.S. Pat. No. 5,039,867 (JP,A 1-151467 and JP,A 1-209077). Also, a process of positioning one X-ray CT unit below a vertical beam delivery unit so as to position on a beam axis of the beam delivery unit and computing the amount of movement of a couch by using a tomographic image (current tomographic image) obtained by the one X-ray CT unit and a tomographic image (reference tomographic image) obtained by another X-ray CT unit before decision of a treatment plan is described from line 67, column 9 to line 18, column 11 (in particular (2a)-(2g)) and FIG. 6 of U.S. Pat. No. 5,039,867.